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public static void main(String[] args) {
Map m  = new HashMap();
m.put(null,"1");
System.out.println("m:"+m);//m:{null=1}
m.put("", null);
System.out.println("m:"+m);//m:{=null, null=1}
m.put(null, null);
System.out.println("m:"+m);//m:{=null, null=null}
Map m1  = new HashMap();
m1.put(null, null);
System.out.println("m1:"+m1);//m1:{null=null}
}